South Korea should join the US BMD in Northeast Asia. Application of Offensive Realism clearly shows that China is a potential threat to South Korea, and Seoul requires an alliance partner in order to counter the threat from the west. The immense latent power and offensive military capabilities of China, coupled with its nuclear strategic weapons render China as a potential threat to South Korean national security. No independent action from South Korea would be sufficient to balance against China, hence the need for an ally. The United States is already engaged in relative power maximization against China. In addition, its role as an offshore balancer and status as a regional hegemon in the western hemisphere makes the US the most ideal alliance partner for South Korea. The alliance with the US to balance against China cannot be built upon the existing ROK-US alliance, because the current status of the alliance has deteriorated. One of the key causes of alliance deterioration could be attributed to the unilateral American security assistance to South Korea. Seoul’s participation in the US BMD would solve the non-reciprocating role of South Korea found in the existing ROK-US alliance. Because the benefits from South Korea’s participation in the US BMD are great for Washington, Seoul would be able to secure the US as an alliance partner to balance against China. Keywords: US, South Korea, China, Ballistic Missile Defense, ROK-US Alliance, Offensive Realism.